Ion implantation apparatus with ion beam directing unit

ABSTRACT

An ion implantation apparatus includes an ion beam directing unit, a substrate support, and a controller. The controller is configured to effect a relative movement between an ion beam passing the ion beam directing unit and the substrate support. A beam track of the ion beam on a substrate mounted on the substrate support includes circles or a spiral.

BACKGROUND

Ion implanters with high current accelerator systems and ion beamcurrents of more than 2 mA emit ion beams with a diameter of severalcentimeters. Several semiconductor substrates may be mounted on asubstrate carrier that rotates during the ion implant. Ion implanterswith medium current accelerator systems and ion beam currents of about 1mA emit ion beams with a diameter of about 1 cm. An electrostaticdeflection system deflects the ion beam along two orthogonal scandirections. For implanting impurities in a semiconductor substrate theion beam linearly scans the semiconductor substrate along parallel linesor zig-zag paths. The implant dose may be locally modified by omittinglines or the line feed for one or more linear scans or by varying thescan speed.

There is a need for improving ion implanters and the methods of ionimplanting.

SUMMARY

According to an embodiment an ion implantation apparatus includes an ionbeam directing unit, a substrate support, and a controller. Thecontroller is configured to effect a relative movement between an ionbeam passing the ion beam directing unit and the substrate supportwherein a beam track of the ion beam on a substrate mounted on thesubstrate support includes circles or a spiral.

According to another embodiment an ion implantation method includesdirecting an ion beam onto a substrate and controlling a relativemovement between the ion beam and the substrate such that a beam trackof the ion beam is a spiral or a circle around a center point of thesubstrate.

According to a further embodiment a semiconductor substrate includes acircular wafer-scale semiconductor body and in the semiconductor body acircular implant zone with a radial variation of doping around a centerpoint of the semiconductor body.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description and onviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification. The drawings illustrate the embodiments ofthe present invention and together with the description serve to explainprinciples of the invention. Other embodiments of the invention andintended advantages will be readily appreciated as they become betterunderstood by reference to the following detailed description.

FIG. 1A is a schematic diagram illustrating an ion implantationapparatus according to an embodiment providing electrostatic deflectionunits for two lateral directions.

FIG. 1B is a schematic plan view of a substrate according to anembodiment with an ion beam track on the substrate including concentriccircles.

FIG. 1C is a schematic plan view on a substrate according to anembodiment with an ion beam track on the substrate including a spiral.

FIG. 1D is a schematic diagram illustrating circular impurity profilesaccording to embodiments.

FIG. 2A a schematic block diagram of an ion implantation apparatusaccording to an embodiment including a rotating substrate support.

FIG. 2B is a schematic plan view of a substrate for illustrating across-sectional line between an ion beam plane and the substrateaccording to another embodiment.

FIG. 3A is a schematic lateral cross-sectional view of semiconductordevice including circular impurity regions according to an embodimentrelated to charge carrier lifetime adjustment.

FIG. 3B is a schematic cross-sectional view of a semiconductor deviceaccording to an embodiment related to a field stop layer with a circularvariation of doping.

FIG. 3C is a schematic cross-sectional view of a thyristor according toan embodiment related to a compensation of a variation of effective gateresistance.

FIG. 4A is a diagram schematically illustrating a lateral profile of thespecific resistance of a semiconductor wafer obtained from a Czochralskiprocess for discussing effects of the embodiments.

FIG. 4B is a diagram schematically illustrating an intrinsic oxygendistribution across a semiconductor wafer obtained from a Czochralskiprocess for discussing effects of the embodiments.

FIG. 4C is a diagram schematically illustrating a charge carrier densityin chip areas of a semiconductor wafer obtained from a Czochralskiprocess for discussing effects of the embodiments.

FIG. 4D is a schematic diagram illustrating impurity distributions forillustrating a method of manufacturing semiconductor devices accordingto a further embodiment.

FIG. 5 is a schematic flow chart of an ion implantation method as wellas a method of manufacturing semiconductor devices according to otherembodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and in which are shownby way of illustrations specific embodiments in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and structural or logical changes may be made without departingfrom the scope of the present invention. For example, featuresillustrated or described for one embodiment can be used on or inconjunction with other embodiments to yield yet a further embodiment. Itis intended that the present invention includes such modifications andvariations. The examples are described using specific language, whichshould not be construed as limiting the scope of the appending claims.The drawings are not scaled and are for illustrative purposes only. Forclarity, the same elements have been designated by correspondingreferences in the different drawings if not stated otherwise.

The terms “having”, “containing”, “including”, “comprising” and the likeare open, and the terms indicate the presence of stated structures,elements or features but do not preclude additional elements orfeatures. The articles “a”, “an” and “the” are intended to include theplural as well as the singular, unless the context clearly indicatesotherwise.

The term “electrically connected” describes a permanent low-ohmicconnection between electrically connected elements, for example a directcontact between the concerned elements or a low-ohmic connection via ametal and/or highly doped semiconductor. The term “electrically coupled”includes that one or more intervening element(s) adapted for signaltransmission may be provided between the electrically coupled elements,for example elements that are controllable to temporarily provide alow-ohmic connection in a first state and a high-ohmic electricdecoupling in a second state.

The Figures illustrate relative doping concentrations by indicating “−”or “+” next to the doping type “n” or “p”. For example, “n⁻” means adoping concentration which is lower than the doping concentration of an“n”-doping region while an “n⁺”-doping region has a higher dopingconcentration than an “n”-doping region. Doping regions of the samerelative doping concentration do not necessarily have the same absolutedoping concentration. For example, two different “n”-doping regions mayhave the same or different absolute doping concentrations.

The ion implantation apparatus 100 in FIG. 1A includes an ion beamsource 110 emitting an ion beam 101. The ion beam source 110 may includean ion source emitting the desired ions, for example protons, helium,donator or acceptor ions such as boron ions, phosphorus ions or arsenicions, by way of example. The ion beam source 110 may further include anaccelerator electrostatically accelerating the ions emitted by the ionsource, a separation magnet for removing undesired impurity ions and alens unit for focusing the ion beam 101. According to an embodiment adiameter of the ion beam 101 is between 0.5 and 2.5 cm.

The ion implantation apparatus 100 further includes an ion beamdirecting unit 120 including a first deflection unit 121 for deflectingthe ion beam 101 along a first lateral direction (x-axis) and a seconddeflection unit 122 for deflecting the ion beam 101 along a secondlateral direction (y-axis) which may be orthogonal to the first lateraldirection. Each of the deflection units 121, 122 may deflect the ionbeam symmetrically with respect to a center position of the ion beam101.

A substrate support 190 fixes a substrate 200 such that a center point105 of the substrate 200 is arranged in the center position of the ionbeam 101.

The substrate 200 may be a circular substrate, for example a standardsemiconductor wafer with any diameter, e.g. at least 25.4 mm. Thesemiconductor wafer may be a silicon wafer, an 301(silicon-on-insulator) wafer, e.g., an SOG (silicon-on-glass) wafer, ora substrate of another single-crystalline semiconductor material such assilicon carbide SiC, gallium arsenide GaAs, gallium nitride GaN, anyother A_(III)B_(V) semiconductor, germanium Ge or a silicon-germaniumcrystal SiGe, by way of example. According to another embodiment, thesubstrate 200 may be a disc-shaped substrate cut out from asemiconductor wafer, e.g. by a laser beam.

The substrate support 190 may align the substrate 200 such that anexposed process surface 201 of the substrate 200 is perpendicular to theion beam 101 in its center position. According to other embodiments, thesubstrate 200 is tilted to the ion beam in its center position by atmost 10 degree, for example by about 7 degrees.

A controller 150 is coupled to the ion beam directing unit 120 andcontrols the deflection of the ion beam 101 to effect a relativemovement between the ion beam 101, which passes the ion beam directingunit 120, and the substrate support 190 such that a beam track 109 ofthe ion beam 101 on the process surface 201 is a spiral or includescircles. For example, the controller 150 may be electrically connectedor coupled to the first deflection unit 121 and to the second deflectionunit 122 and effects a modulation of a deflection voltage applied to thefirst and second deflection units 121, 122 in an appropriate way.

As illustrated in FIGS. 1B and 1C each of the deflection units 121, 122may include two deflection plates 121 a, 121 b, 122 a, 122 b on opposingsides of the ion beam 101, respectively. The control unit 150 may applya first deflection signal s1(t) to the first deflection unit 121 and asecond deflection signal s2(t) to the second deflection unit 122.According to the illustrated embodiment the first deflection signals1(t) is applied to a first deflection plate 121 a of the firstdeflection unit 121 and the second deflection signal s2(t) is applied tothe first deflection plate 122 a of the second deflection unit 122,whereby the second deflection plates 121 b, 122 b are grounded.According to other embodiments the controller unit 150 may applydifferential deflection signals to both deflection plates of eachdeflection unit 121, 122.

The amplitudes of the two deflection signals s1(t), s2(t) may fluctuatearound mean values defining a center position of the ion beam 101. Thetwo deflection signals s1(t), s2(t) may be sinusoidal signals, which mayhave the same frequency and which may be phase-shifted to each other byπ/2 such that the ion beam follows the surface of a cone and for eachcomplete oscillation the ion beam 101 surrounds the center point 105once between the center point 105 and a lateral area 203 tilted to theprocess surface 201. The amplitudes of the two deflection signals s1(t),s2(t) determine the distance of the circulation to the center point 105.

According to an embodiment with the process surface 201 perpendicular tothe ion beam 101 in its center position the amplitudes of the twodeflection signals s1(t), s2(t) may be equal such that for each completeoscillation the beam track 109 on the process surface 201 is a circle.According to embodiments with the process surface 201 tilted to the ionbeam 101 in its center position the amplitude ratio of the twodeflection signals s1(t), s2(t) may be adjusted in a way such that thebeam track 109 includes circles around the center point 105.

For achieving a homogeneous implant dose across the process surface 201the amplitudes of the deflection signals s1(t), s2(t) may be variedproportional to 1/r at a fixed amplitude ratio, with r indicating thedistance of the ion beam 101 to the center point 105 on the processsurface 201. Starting from deflection signals for homogeneous implantdoses rotational-symmetric, e.g., circular implantation profiles may beobtained by modifying, at the same implant current, the frequency of thefirst and second deflection signals s1(t), s2(t) resulting in amodification of the implant dose along the beam track 109, or bymodifying, at the same amplitude ratio, the amplitude gradient of thedeflection signals s1(t), s2(t) resulting in a radial modification ofthe density of circulations of the beam track 109, by way of example.

According to the embodiment of FIG. 1B the amplitudes of the deflectionsignals s1(t), s2(t) are constant during one or more completecirculations and are simultaneously changed, at the same amplituderatio, after the beam track 109 has completed one or more completecirculations. The beam track 109 includes circles of different diametersand the impurity distribution is circular. According to anotherembodiment the frequency of both deflection signals s1(t), s2(t) mayvary for only a portion of a circle such that the implant dose variesalong the beam track 109.

According to the embodiment of FIG. 1C the controller 150 of FIG. 1Asteadily increases the amplitudes of both deflection signals s1(t),s2(t) such that the beam track 109 of the ion beam 101 on the processsurface 201 is a spiral. The controller 150 may control the deflectionunits 121, 122 such that the beam track 109 winds inwardly or outwardly.The beam track 109 may cross the whole substrate 200 between the centerpoint 105 and a lateral area 203 or a portion thereof. The frequency ofboth deflection signals s1(t) s2(t) may vary for only a portion of acirculation such that the implant dose varies along the beam track 109.

FIG. 1D shows impurity concentration profiles in the semiconductorsubstrate 200 which can be obtained by using the ion implantationapparatus 100 of FIG. 1A. For profile 302, the beam track 109 crossesonly an inner portion including the center point 105, for profile 304the beam track 109 crosses an outer portion along the lateral area 203and for profile 306 a ring portion distant to both the center point 105and the lateral area 203. The ion implantation apparatus 100 is adaptedto form rotational-symmetric, e.g., circular impurity regions and avariation of doping along circles or a spiral.

Where for circular impurity profiles conventional ion implantersperforming linear line-by-line scanning along the x-axis or alongzig-zag paths require rather complex and time-consuming programmingsteps for locally varying the implant dose by modifying the scans, theion implantation apparatus 100 achieves a similar effect at less effortand with better local resolution.

The ion implantation apparatus 100 of FIG. 2A includes an ion beamsource 110 emitting an ion beam 101 and an ion beam directing unit 120that deflects the emitted ion beam 101 along a linear direction, e.g.,along the x-axis. The ion beam directing unit 120 may include adeflection unit with two deflection plates 120 a, 120 b. In addition, amotor drive unit 180 rotates the substrate support 190 around a centerpoint 105 of a substrate 200 fixed on the substrate support 190 in aplane that is perpendicular to the ion beam 101 in its central positionor slanted by at most 10 degree, for example by about 7 degree withrespect to the plane perpendicular to the ion beam 101 in its centerposition.

The controller unit 150 may control the ion beam direction unit 120 suchthat the ion beam 101 moves in a plane spanned by a normal to theprocess surface 201 or tilted to the normal by a tilt angle of at most10 degree, for example about 7 degree. In addition, the controller 150may detect and/or control the rotational speed of the substrate support190 by monitoring or controlling the motor drive unit 180 whose shaft iscoupled to the substrate support 190. Section line 108 indicates thecross-sectional line between the plane in which the ion beam 101 movesand the process surface 201 of the substrate 200. The section line 108includes the center point 105.

The deflection signal s3(t) may be a linearly decreasing or increasingsignal 201. For a constant rotational speed or the substrate support 190the deflection signal s3(t) may change only slowly close to the lateralarea 203 and faster with decreasing distance to the center point 105 asshown in FIG. 2B. For example, with r indicating the distance to thecenter point 105 the deflection signal s3(t) may change at a rate 1/r asindicated by signal 202 for achieving a homogeneous impuritydistribution.

According to other embodiments the deflection signal s3(t) may be anamplitude modulated periodic signal 203, wherein the frequency of theperiodic signal 203 may be lower than the rotational rate of thesubstrate support 190 also as shown in FIG. 2B. According to anembodiment the rotational rate is at least ten times as high as thelinear scan rate of the deflection signal s3(t). During each completelinear scan the ion beam 101 may circulate at least ten times around thecenter point 105 for achieving a sufficient homogeneous impurityprofile.

FIGS. 3A to 3C refer to wafer-scale semiconductor devices 200 x obtainedby using the ion implantation apparatuses 100 of FIGS. 1A and 2A oncircular semiconductor substrates. The wafer-scale semiconductor devices200 x are based on cylindrical semiconductor bodies 205 whose diametermay correspond to a standard wafer diameter, e.g., 25.4 mm, 50.8 mm,76.2 mm, 100 mm, 125 mm, 150 mm, 200 mm, or 300 mm or any other valuegreater than 10 mm. A wafer-scale semiconductor device 200 x is a singlesemiconductor die and not integrated in a wafer composite that includesa plurality of identical semiconductor dies.

The semiconductor device 200 x of FIG. 3A may be a disc-shaped thyristoror a disc-shaped semiconductor diode with approximately circular lateralbase areas 201, 202. In a thyristor on-state or in the forward mode of asemiconductor diode mobile charge carriers flood a drift one of thesemiconductor body between the two opposing base areas 201, 202. Whenthe thyristor changes to an off-state or blocking state or when thesemiconductor diode changes to a blocking mode, the respectivesemiconductor device 200 x commutates wherein the mobile charge carriersare removed from the drift zone. Typically an edge area 290 directlyadjoining a lateral area 203 connecting the base areas 201, 202 is lesseffective in removing the mobile charge carriers from the drift zone,because, for example, electrode structures do not reach the lateral area203.

In the semiconductor device 200 x a circular implant of impurities witha radial concentration variation may result in a ring-shaped circularimplant zone 295 in the edge area 290 around a center point 105, whereinthe circular implant zone 295 may be spaced from the lateral surface 203or may directly adjoin the lateral surface 203. In the circular implantzone 295 implanted ions such as protons or helium (He) ions may generateparticle-induced crystal defects in the silicon crystal lattice. Thecrystal defects significantly reduce the charge carrier life time in thecircular implant zone 295 such that less charge carriers have to beremoved from the edge area 290 when the semiconductor device 200 xcommutates. As a result the dynamic losses in the semiconductor device200 x are lower than without the circular implant zone 295.

The implantation-induced defect concentration in the circular implantzone 295 may continuously or in steps increase in radial direction withincreasing distance to the center point 105. Forming the circularimplant zone 295 by the circular implantation method described above maysave a graded implant mask and may achieve better circular uniformitythan unmasked implants based on linear scans.

The semiconductor device 200 x of FIG. 3B may be a semiconductor diodeor a thyristor including a heavily doped pedestal layer 252 forming anohmic contact with a metal-containing rear side electrode 251, a lowdoped drift zone 255 as well as a field stop layer 253 between the lowdoped drift zone 255 and the pedestal layer 252.

The pedestal layer 252 may have a central portion 252 a having a highpartial transistor gain around the center axis 207 and a circular outerportion 252 b with low partial transistor gain along the lateral area203. At lower partial transistor gain less charge carriers are injectedinto the edge area 290 such that less charge carriers have to be removedfrom the edge area 290 during commutation. Reducing the number of chargecarriers during commutation in the edge area 290 also increases thedynamic dielectric strength of the edge area 290 and avalanche breakdownmainly occurs between the edge area 290 and the center axis 207.

The field stop layer 253 may be a circular implant zone with a circularimpurity profile including a circular first portion 253 a around thecenter point 105, a circular second portion 253 b enclosing andsurrounding the first portion 253 a and a circular third portion 253 cbetween the second portion 253 b and the lateral area 203.

The field stop layer 253 may be formed by circular implantation of ionssuch as protons that generate circularly-distributed radiation-inducedcrystal defects. During an anneal at temperatures between 270° C. and500° C. particle-related donors such as hydrogen-related donors may format the radiation-induced crystal defects, wherein the hydrogen-relateddonors may be hydrogen-decorated intrinsic point defect complexes.

In case the semiconductor device 200 x is a thyristor, the pedestal andfield stop layers 252, 253 form a pn-junction. The impurityconcentration in the third portion 253 c of the field stop layer 253 ishigher than the impurity concentration in the second portion 253 b. Theimpurity concentration in the second portion 253 b is higher than theimpurity concentration in the first portion 253 a. The higher dopedportion 253 c prevents a depletion zone associated with an avalanchecurrent from a critical punch through to the rear side electrode 251 inthe edge area 290.

The circular patterns of the pedestal layer 252 and the field stop layer253 result in high dynamic robustness and high avalanche currentstrength.

In case the semiconductor device 200 x is a semiconductor diode thepedestal and field stop layers 252, 253 form a unipolar homojunction,e.g. an nn⁺ or pp⁺ junction. The impurity concentration in the thirdportion 253 c of the field stop layer 253 is lower than the impurityconcentration in the second portion 253 b. The impurity concentration inthe second portion 253 b is lower than the impurity concentration in thefirst portion 253 a. In addition to the impurity concentration, thevertical extensions of the first, second, and third portions 253 a, 253b, 253 c may be varied.

The circular pattern of the field stop layer 253 may be adjusted toincrease the blocking capability in the edge area 290.

In FIG. 3C the semiconductor device 200 x is a GTO (gate turn-off)thyristor with a semiconductor body 205 including a p-type pedestallayer 253 effective as anode layer and forming an ohmic contact with arear side electrode 251 effective as anode electrode, an n-type base 255forming a pn junction with the pedestal layer 253, a p-type base layer257 forming a pn junction with the n-type base layer 255, and heavilyn-type cathode zones 259 forming pn junctions with the p-type base layer257. On a first base area 201 portions of a cathode electrode 258 formohmic contacts with the cathode zones 259 and portions of a gateelectrode 256 form ohmic contacts with the p-type base layer 257.

When a current supplied through the gate electrode portions 256 exceedsa threshold value, ignition starts close to the respective gateelectrode portion 256 and propagates to the center of the cathode zones259. For turning the GTO off, a negative gate current is suppliedthrough the gate electrode portions 256 and within the p-type base layer257 holes flow in radial direction to the gate electrode portions 256.The time for switching off a cathode zone 259 depends on the impurityconcentration in the p-type base layer 257 and the timing of the gatesignal at the gate electrode portions 256. Due to the radial variationof the ohmic line resistance, the gate signal may be delayed betweengate electrode portions 256 closer to a supply point of the gate signaland gate electrode portions 256 more distant to the supply point.

A suitable radial variation of a defect concentration with a defectdensity increasing in radial direction and hence a radial decrease ofthe charge carrier life time in the n-type base zone 257 may compensatefor the radial variation of the effective resistance of the gatemetallization and the gate signal propagation delay between ring-shapedgate electrode portions 256. With a defect density increasing in radialdirection the cathode zones 259 are switched off within a narrower timespan and a maximum current which the thyristor can switch off may besignificantly increased.

Generally, a radial variation of defect and/or dopant concentrationprofiles may be used for a better tradeoff between the static blockingcapability of semiconductor devices and the softness of a currentgradient during switching off.

FIGS. 4A to 4D refer to embodiments applying a circular ion implantationto semiconductor substrates such as semiconductor wafers.

Silicon semiconductor wafers, e.g., wafers with a diameter of 300 mm,may be obtained from molten silicon using a Czochralski process. A rodwith a seed crystal dips into the molten silicon. Single-crystallinesilicon crystallizes at the seed crystal and when the rod is slowlypulled out of the melt, the silicon crystal forms a cylindrical ingotwhich follows the rod. The rod may be rotated during the crystal growthprocess. M:Cz silicon wafers are obtained from a magnetic Czochralskiprocess using an external magnetic field that suppresses or controls amelt flow in the molten silicon.

Cz and m:Cz wafers may contain interstitial oxygen atoms which incombination with crystal defects may be effective as donors. Typically,a high energetic proton implant is used to generate a homogeneousbackground donor concentration in at least a partial layer of thesemiconductor wafer, e.g. in a layer that corresponds to a lightly dopeddrift zone of the finalized devices. The implanted protons may interactwith the intrinsic oxygen and with crystal defects generated by theimplant to generate a doping effective as donors. The intrinsic oxygendistribution in a virgin m:Cz wafer significantly affects the localefficiency of the proton implant as well as the specific ohmicresistance distribution.

It could be demonstrated by the inventors that the Czochralski m:Czprocesses result in approximately circular inhomogeneities of the oxygendistribution in the silicon ingot and the semiconductor wafers obtainedfrom the silicon ingot.

FIG. 4A shows the specific ohmic resistance distribution in an m:Czsilicon wafer with a radius R. The specific resistance includes maximaand minima in an approximately circular pattern with respect to a centerpoint at r=0. The variation of the specific resistance is about 15% of amean value in a central region around the center point.

FIG. 4B schematically shows the intrinsic oxygen density distribution inan m:Cz silicon wafer obtained from an ingot manufactured in a magneticCzochralski process. The oxygen concentration in a center portion isabout 11% higher than along the lateral area of the semiconductor wafer.

FIG. 4C illustrates rectangles assigned to semiconductor dies obtainedfrom a single m:Cz silicon wafer, wherein dark rectangles indicate highcharge carrier concentrations and light rectangles indicate low chargecarrier concentrations. Yet at the very beginning of manufacturing,semiconductor dies assigned to different wafer areas differsignificantly with respect to the background charge carrierconcentration. Though obtained from the same m:Cz silicon wafer, suchsemiconductor devices obtained from an edge region of the m:Cz siliconwafer may have charge carrier concentrations in low-doped regions, e.g.drift zones, which significantly deviate from the charge carrierconcentrations in semiconductor devices obtained from a central regionof the same m:Cz silicon wafer. The differences add to the fluctuationsin further processes.

Implantation of ions such as protons using a circular regime maycompensate for circular distributions of the oxygen distribution. Forexample, when the semiconductor wafer receives a background impurityconcentration, a suitable implant of protons may compensate for thedifferences in the background concentration.

For conventional ion implanters which typically modify a local impurityprofile in an x-y grid a laborious and complex programming compensatesfor the inherent impurity concentration in the virgin Czochralski wafer.On the other hand, the circular implantation scheme as described aboveprovides a simple and cost-effective method for compensating circularprocess inhomogeneities introduced, e.g., by the Czochralski process,e.g. in wafers having a diameter of 300 mm and more.

Accordingly, a method of manufacturing semiconductor devices may includepulling a single crystalline silicon ingot from fluidified silicon in aCzochralski process and obtaining, for example by cutting or sawing,semiconductor wafers from the silicon ingot.

The distribution of the resulting effective charge carrier concentrationin the semiconductor wafer is determined, for example by measurement,and may be approximated by a circular approximation. Then a compensationprofile for compensating the approximated circular impurity distributionmay be determined and transferred to the controller unit 150 of the ionimplantation apparatus 100 of FIG. 1A or 2A. The background implant isperformed, wherein uniformity of the resulting impurity concentrationprofile of the Czochralski wafer is improved.

FIG. 4D schematically shows the radial oxygen distribution 312 in avirgin Czochralski m:Cz silicon wafer, the circular approximation 314,the profile of the compensation implant 316 using protons and theresulting effective donor profile 318. Where the initial oxygenconcentration and the concentration of oxygen-induced thermal donors arelow, more protons are implanted than where the initial oxygenconcentration and the concentration of oxygen-induced thermal donors arehigh.

The compensation implant 316 forms a circular implant zone containinghydrogen-related donors. A radial concentration variation of thecompensation implant 316 around the center point of the semiconductorwafer may compensate to some degree a radial variation of theconcentration of the oxygen-induced donors. The resulting effectivedonor profile 318, which is the sum of the concentrations of thehydrogen-related donors and the oxygen-induced thermal donors, can betailored such that a variation of the sum of the concentrations is atmost 25%, e.g. at most 15% or even below 10%.

The circular implant compensates for effects caused by the oxygendistribution.

FIG. 5 refers to an ion implantation method as well as to a method ofmanufacturing a semiconductor device. An ion beam is directed on asubstrate (502). A relative movement between the ion beam and thesubstrate is controlled such that a beam track of the ion beam on asurface of the substrate is a spiral or includes a circle around thecenter point of the substrate (504).

A method of manufacturing a semiconductor device includes directing anion beam onto a substrate and controlling a relative movement betweenthe ion beam and the substrate such that a beam track of the ion beam isa spiral or includes circles. A single-crystalline silicon con ingot maybe pulled out from fluidified silicon and the substrate may be obtainedby sawing the silicon ingot orthogonal to its longitudinal axis.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. An ion implantation apparatus, comprising: an ionbeam directing unit; a substrate support; a controller configured toeffect a relative movement between an ion beam passing the ion beamdirecting unit and the substrate support, wherein a beam track of theion beam on a substrate mounted on the substrate support includescircles or a spiral in a plane; and a controllable motor drive unitconfigured to rotate the substrate support in a plane slanted by at most10 degree with respect to a plane perpendicular to the ion beam.
 2. Theion implantation apparatus of claim 1, wherein the controller isconfigured to control a rotational speed of the substrate support. 3.The ion implantation apparatus of claim 2, wherein the ion beamdirecting unit comprises an electrostatic deflection unit configured todeflect the ion beam along a first direction.
 4. The ion implantationapparatus of claim 3, wherein a track of the ion beam intersects arotational axis of the substrate support.
 5. An ion implantationapparatus, comprising: an ion beam directing unit; a substrate support;and a controller configured to effect a relative movement between an ionbeam passing the ion beam directing unit and the substrate support,wherein a beam track of the ion beam on a substrate mounted on thesubstrate support includes circles or a spiral in a plane, andconfigured to at least one of radially modifying a distance betweensuccessive beam tracks and radially modifying a rotational speed of theion beam track to obtain a rotational-symmetric implant dose withradially varying implant dose.